1. Technical Field of the Invention
The present invention concerns a molecular vacuum pump for evacuating a gas from a chamber, thereby generating a high vacuum which is generally situated between 0,1 mbar and 10xe2x88x928 mbar and preferably between 10xe2x88x922 and 10xe2x88x926 mbar.
2. Prior Art
At present, when one wishes to maintain a pressure in the order of magnitude of 10xe2x88x925 to 10xe2x88x927 mbar, for example in order to evaporate a material under vacuum, or when it is necessary to maintain pressures in the order of magnitude of 10xe2x88x922 to 10xe2x88x925 when carrying out a plasma process for example, use is made of molecular pumps which can work under said pressure range. The required pressure of equilibrium is obtained by establishing the equilibrium between the inlet yield of the gases and the pump discharge of the system at a certain level. The pumping speed of the pump is generally fixed (volume per unit of time), the pressure is regulated by regulating the leakage rate in the vacuum chamber. It should be noted that when no gas is introduced, the pressure does not fall indefinitely, but it reaches what is called the limiting pressure of the system, resulting from the equilibrium between the leaks which are inherent to every installation and the speed of the installed pumping unit.
For the above-mentioned pressure range, two types of pumps are used in practice: what are called diffusion pumps on the one hand, based on the drag-in of the gas from the chamber in which the vacuum is to be created, by the ejection of gas by means of a series of concentric nozzles which are incorporated in the body of the pump, and molecular rotary entrainment pumps on the other hand (molecular turbopump and xe2x80x9cmolecular drag pumpxe2x80x9d) which drag the gas molecules colliding with the rotor of the pump. Both types of pumps represent major disadvantages, however.
In a diffusion pump, as use is made of fluids to be evaporated such as hydrocarbons and silicones, whose vapours drive the pumping, problems arise in that the chamber in which the vacuum is to be created is contaminated due to the reverse diffusion of the vapours of the pump in the chamber. Moreover, large amounts of energy and water are consumed for the evaporation and condensation of said fluids. Further, a diffusion pump must be compressed to a large extent in order to be able to function at a pressure which is superior or equal to 10xe2x88x923 mbar in the chamber, in order to avoid major pressure variations and an important contamination of the vacuum chamber. Generally, said compression strongly reduces the pumping speed of the pump.
Besides, a rotary molecular pump is only efficient when the rotational speed of the rotor is of the same order of magnitude as the speed at which the gas molecules are moved, which implies very high rotational speeds, generally situated between 30,000 and 80,000 revolutions per minute, depending on the size of the pump. Only at such rotational speeds can the far end of the rotor reach its maximum speed, which is in the order of magnitude of 500 m/sec for the best pumps. A speed increase is not easy to realise, given the mechanical difficulties which need to be overcome. At such speeds, the rotor, which is generally made of an aluminium alloy, is subjected to major stress conditions of up to 150 N/mm2. Hence, it is very important, in order to prevent the rotor from crashing against the stator, that the rotor is perfectly positioned (to a xcexcm) by means of advanced methods for dynamic balancing under vacuum at nominal speed. The tooling and especially the balancing thus represent a very high cost in the cost price of a rotary molecular pump. Of all the problems related to the use of a rotary molecular pump, we should mention in particular:
the considerable wear of the mechanical bearings usually makes it necessary to use magnetic or gas bearings, which are very expensive;
when mechanical bearings are used, the use of lubricant may result in a contamination of the chamber which, although it is negligible compared to with what is obtained with a diffusion pump, may be critical in certain applications;
when there is a magnetic field superior to 10 mT, the use of rotary molecular pumps with a conductive rotor is seriously complicated by the presence of induced currents which will overheat the latter;
increasing the pumping speed of this type of pump is difficult and expensive above 5000 l/sec because of the equipment which is required for tooling and balancing these pumps.
The present invention mainly aims to provide a molecular pump which makes it possible to remedy the disadvantages of the existing pumps of this type.
To this end, the pump according to the invention comprises a substantially sealed box having on one of its sides an intake orifice to be connected to said chamber and, on the opposite side, an outlet orifice, to be preferably connected to a discharge pump, whereby elements are mounted between those two orifices at some distance from one another in substantially fixed sites inside said box for the gas to pass through, said elements being of such a nature as to impart to said gas molecules, coming from said chamber and coming into contact with said elements, a speed whereof the resultant is oriented towards the outlet orifice.
Advantageously, said elements work in conjunction with means which make it possible to subject them to a vibration having a component which is directed towards the outlet orifice.
According to a preferred embodiment of the pump according to the invention, the above-mentioned element contains a piezo-electric material fixed on the above-mentioned support and coated, on the side opposite to the one which is directed towards the support, with an electrically conductive coating, whereby means are provided to apply an alternating current to said element, such that said piezo-electric material is subjected to a deformation in a direction transversal to the support and, consequently, said coating is exposed to a corresponding vibration.
Other particularities and advantages of the pump according to the invention will become clear from the following description, in which some particular embodiments of this pump are represented as an example only and without being limitative in any way, with reference to the accompanying drawings.